Raw sugar is made from sugar cane in a series of steps including the grinding of the cane from which raw juice is extracted. This raw juice is clarified through a combination of physical and chemical treatments such as carbonation and sulfitation. The resulting clarified juice undergoes a series of treatments (evaporation, crystallization and centrifugation) to obtain a raw sugar (in the form of sugar crystals that present a yellow color).
This raw sugar is then sent to a refinery where it is cleaned, purified and made ready for the consumer. The refining conventionally consists of a series of treatment steps where a raw sugar liquor is formed of the raw sugar, and color and non-sugar impurities are gradually eliminated from this liquor and pure sugar is produced. The refining process can produce a clear solution of sugar in water called liquid sugar, or solid refined sugar after crystallization.
The first step of the refining process is aimed at removing solids present in the raw sugar liquor while subsequent steps are performed to decolorize the liquor. The lightly colored liquor normally undergoes crystallization but it has to be evaporated prior to crystallization because it is too dilute. The raw sugar liquor can also be used to produce liquid sugar, and in this case it requires a final activated carbon filtration to remove turbidity and it normally has to be disinfected.
This series of steps in the refining stage has conventionally required costly installations such as a flotation system, filtration devices, activated carbon or resin columns, and regeneration installations. It also imposes high cost in the chemicals needed to remove solids and colored compounds. The disinfection of sugar liquors has conventionally been realized through the addition of biocides or antibiotics that also involve high costs.
The liquor that enters the refining stage still contains color compounds (by which is meant compounds that impart color to the sugar), and other fine particles, gums and resins. In conventional processing, the first step of processing the liquor is known as phosphatation where lime (or calcium saccharate) and phosphate are added to form a calcium phosphate (hydroxyapatite). That mineral acts as a matrix for entrapment of color compounds and other non-sugars. The thus-collected impurities are eliminated by filtration or flotation. Flotation is a more efficient technique. In that case air is injected and some polymers such as polyacrylamide are added to improve the elimination of impurities. These impurities are recycled because they still contain sugar that can be recovered.
The sugar liquor is then decolorized. Conventionally, this is done in refineries by absorption techniques wherein the liquor is pumped through columns of absorption medium (carbon or resins). The most common medium is based on the use of granular activated carbon but another option is to use an ion exchange resin. The carbon is regenerated in a hot kiln or simply discarded while the resin is regenerated chemically but produces liquid effluents.
The whole refinery sequence is considered as very effective to remove color and to produce a sugar of high quality, but it requires costly installed equipment, and high operational cost due to the use of chemicals and filtration medium.
It has been proposed in e.g. Davis et al., xe2x80x9cThe Use of Ozone for Colour Removal at the Malelane Refineryxe2x80x9d, Proc. S. Afr. Sug. Technol. Ass. (1998) 72, pp. 255-260, to decolorize sugar liquors by a method which includes the treatment of the sugar melt with ozone in a high shear mixer followed by carbonation of the liquor. According to this disclosure, the use of ozone in a single step is not sufficient to reach the required color reduction, and a carbonation treatment step is indispensable.
The proposed invention is based on a simple process that will carry out the clarification and disinfection in one step. A second step may be included to remove the increase of turbidity.
The process comprises
(a) providing a raw sugar solution having color of 50 to 1,000 Icumsa units and a concentration of 10 to 70 Brix, wherein said raw sugar solution contains particles imparting color to the solution,
(b)(1) injecting ozone into the raw sugar solution while imparting agitation to the solution effective to reduce the size of said color-imparting particles into smaller particles, and then
(b)(2) discontinuing said injection, but continuing said agitation for a period of time in excess of the duration of step (b)(1), wherein said smaller particles react with ozone, and
(c) optionally repeating steps (b)(1) and (b)(2) at least once, wherein the solution is at no point subjected to a precipitation or carbonation step.
Ozone is applied into the raw sugar liquor in a mixing reactor, preferably a gas-liquid mixing system such as the xe2x80x9cAdvanced Gas Reactorxe2x80x9d or AGR reactor sold by Praxair, Inc. and described in U.S. Pat. No. 5,244,063. Ozone is applied into the liquor in one or several injections (generally 1 to 6 and preferably 3 or 4) spaced apart in time.
During the ozone injection, and between each injection, and optionally but preferably also before the first ozone injection, mechanical agitation is imparted to the solution and maintained. The mechanical agitation reduces the size of the color-imparting particles to smaller particles.
This process clarifies and disinfects the sugar liquor. In an additional step, if desired, the material is filtered using, for example, a microfiltration device or other apparatus effective to remove turbidity from the solution.
Compared to the prior art this invention lowers the capital investment and operational costs required to produce clarified sugar.
In other aspects of the present invention, the process is carried out at 50 to 80xc2x0 C., and the pH of the solution is maintained at 6.5 to 7.5.
The technical advantages of the process of the present invention include the decolorization and disinfection of sugar liquors with ozone to acceptable levels without the need for an additional carbonation or precipitation step as taught in the prior art.
Sugar liquors usually contain a few crystals, vegetal fibers and pieces of bark and a lot of colored objects. Some are clusters of smaller structures that present a dark blue color under microscopic optical observation (1000 xc3x97magnification). The average size of these clusters is about 10 square microns. The mechanical action provided for instance with a propeller type mixer such as the AGR, is necessary to break up these particles so that adding in the ozone realizes efficient oxidation for removing color. A simple contact column is not effective to reduce significantly the color of sugar liquor at the commonly encountered densities.
After ozonation accompanied by mechanical agitation as in the present invention, microscopic observation indicates that clusters are not present anymore but a lot of small components, much smaller, have been created in the medium. These small objects are uniformly scattered in the solution and they are responsible for an increase of turbidity. For some specific applications of liquid sugar this turbidity must be eliminated through simple microfiltration techniques.
Therefore, the decolorization process is based on a combination of a mechanical action such as that of propeller type device such as the AGR mixer and a powerful oxidation with ozone to obtain an effective color removal, followed preferably by a microfiltration step to remove turbidity.
The quantity of ozone needed to remove color depends on the initial color of the liquor and the required level of final color. This means that the ozone dosage should and can be determined experimentally for each type of sugar liquor. It is generally simpler, industrially, to operate in a batch mode than in a continuous process.
It has been discovered that considering the physical nature of colored compounds, ozone has to be applied in a reactor that will thoroughly mix the gas with the sugar. A propeller type reactor such as the AGR mixer can be used. It was observed that a better efficiency is obtained if ozone is injected directly into the liquor below the propeller, in the vortex zone.
The duration of an ozone injection step, and the length of time between injections of ozone while agitation continues, can vary somewhat depending on the sugar concentration, the degree of color before treatment begins, and the quantity of raw sugar liquor being treated. However, as a general guide, each ozone injection can last on the order of up to 5 minutes, preferably up to 2 minutes and more preferably up to 1 minute. When an ozone injection ceases, it is preferred that agitation is continued for longer than the injection lasted. Generally, agitation after an injection of ozone lasts on the order of up to 10 minutes, such as 5-8 minutes. A total of 1 to 6 ozone injections is generally adequate.
Preferably, the present invention includes
Controlling the pH of the liquor to a value of 6.5 to 7.5, preferably 7.0, by adding a small volume of a concentrated alkali (for example 2 millimoles of caustic soda per liter of sugar liquor at Brix 66)
Operating at temperature above 50xc2x0 C. (but below 80xc2x0 C.), preferably applying the ozone to the sugar liquor at about 700xc2x0 C. The liquor can be prepared at this temperature and sent to the reactor. It is not necessary to maintain the liquor at the same temperature during the ozonation process.
Filtering the liquor after ozonation to remove turbidity, if desired, by a simple microfiltration step.
The proposed invention is applicable to all types of sugar liquors and melts that present a color between about 50 to 1000 Icumsa units, at a concentration between about 10 to 70 Brix. These sugar liquors may come from the refinery step or from another part of the process of sugar fabrication or alcohol production.
The ozone used for sugar liquor clarification and disinfection may be produced from an on-site generation system at concentrations between 3 to 15% in the gas stream. According to the initial level of color, ozone doses will vary in the range from 200 to 2000 g/L of sugar liquor at common Brix of 10-70.